The present invention relates to fused-fiber wavelength division multiplexers (WDM) and, in particular, to a multi-window dense WDM structure.
With existing fiber optic networks, there is often the need to increase information transmission capacity. However, both physical and economic constraints can limit the feasibility of increasing transmission capacity. For example, installing additional fiber optic cable to support additional signal channels can be cost prohibitive, and electronic system components may impose physical limitations on the speed of information that can be transmitted. The use of wavelength division multiplexers (WDMs) provides a simple and economical way to increase the transmission capacity of fiber optic communication systems by allowing multiple wavelengths to be transmitted and received over a single optical fiber through signal wavelength multiplexing and demultiplexing. In addition, WDMs can be used in fiber optic communication systems for other purposes, such as dispersion compensation, noise reduction, and gain flatting, i.e., maintaining a uniform gain within the usable bandwidth for erbium doped amplifiers.
WDMs can be manufactured using, for example, biconical tapered fusion (BTF) technology. Typically, two optical fibers are fused together along an interior portion to form a fused-fiber coupler, so that light of two wavelengths (i.e., 1310 nm and 1550 nm) entering the input terminals of the first and second fibers, respectively, are multiplexed onto a single fiber. The coupling ratios for the two channels (the signals at 1310 nm and 1550 nm) exhibit complementary sinusoidal behavior for amplitude as a function of frequency within the passband of the WDM, with each channel having one or more peaks (or windows) within the passband. Information carried by the two signals along the single fiber is then demultiplexed at the WDM outputs. Light at 1550 nm is particularly desirable because minimal absorption is exhibited by optical fibers around this wavelength. Commercially available fused-fiber WDMs typically also couple and decouple light at 1550 nm and 980 nm and at 1550 nm and 1480 nm.
The principles of WDM can be extended to further increase data transmission capability by coupling additional discrete wavelengths or channels onto a single fiber using devices known as dense WDMs (DWDMs). DWDM is a one-to-N device, as shown in FIG. 1. Fused-fiber DWDM 10 may couple N discrete communication channels xcex1 through xcexN onto a single optic fiber xcex. For example, 8, 16, or even 32 discrete communication channels may be coupled onto a single optic fiber. However, because the usable bandwidth of the light is limited, increasing the number of wavelengths necessarily results in smaller channel separation between the discrete wavelengths. In general, smaller channel spacing can be achieved by increasing the length of the fused portion of a fused-fiber DWDM. However, decreasing channel spacing presents different types of problems, such as increased sensitivity to temperature fluctuations.
A DWDM may comprise several or a plurality of multi-window WDMs (MWDMs). An MWDM is a one-to-two device, as shown in FIG. 2. Light with wavelength xcex enters MWDM 20 which decouples wavelength xcex into two groups, one consisting wavelengths xcex1 to xcexNxe2x88x921 and one group consisting wavelengths xcex2 to xcexN, where N is an even number.
MWDMs have two or more peaks of amplitude as a function of frequency (or operational windows) for each channel within a passband. MWDMs can also be made using BTF technology by putting two optical fibers in parallel, fusing the center portion together, and pulling the fibers until a desired multi-window transmission spectrum appears at a monitored fiber output terminal.
Fused-fiber couplers generally exhibit polarization-dependent loss (PDL). This PDL is induced by the difference of two polarization-dependent coupling coefficients in the tapered regions of the coupler, where two optical fibers are fused together and elongated for optical power coupling. The cross-sectional shape of the tapered section is elliptical or dumbbell-shaped which produces birefringence along the tapered section.
FIG. 3 illustrates spectral transmitted ratios for two polarizations, e.g., x-polarization and y-polarization. Curve 30, representing, e.g., x-polarization, is shifted from curve 31, representing, e.g., y-polarization, in terms of wavelength. The difference of transmitted power for the two modes at the same wavelength xcex1 is defined as the PDL between x-polarization curve 30 and y-polarization curve 31. Curve 30 is shifted from curve 31 because the two principle polarizations x and y have different polarization coefficients. Hence, a light propagating along the x-polarization sees a different geometrical structure than a light propagating along the y-polarization, thus, a difference in the coupling strength. PDL is undesirable because it limits the system performance, e.g., less passbands and larger signal fluctuation.
One way to reduce the polarization sensitivity of BTF MWDM is to use longer pull length to reach optimum phase match condition between the propagation mode of light. Such a long-tapered-fusing technology is discussed in commonly-owned U.S. Pat. No. 5,809,190, entitled xe2x80x9cApparatus and Method of Making a Fused Dense Wavelength-Division Multiplexerxe2x80x9d, which is incorporated by reference herein in its entirety. However, methods of quenching the polarization sensitivity generally lead to an increased temperature sensitivity. In addition, this method requires longer pull length, which reduces the diameter of the tapered section and increases the chance of breakage.
Another method, a paper by I. J. Wilkinson and C. J. Rowe entitled xe2x80x9cClose-Spaced Fused Fibre Wavelength Division Multiplexers with Very Low Polarisation Sensitivityxe2x80x9d (the Wilkinson paper), Electronics Letters, vol. 26, No. 6, pp. 382-384, Mar. 15, 1990, describes how the polarization sensitivity (birefringence) of a wavelength multiplexing fused fiber 2xc3x972 coupler can be substantially nulled-out by elasticity twisting the coupler after its fabrication. However, this paper fails to provide precise wavelength spacing control during the device fabrication, which is critical to a MWDM. The precise control of channel spacing is critical because of the cumulative effect of channel spacing offset across the operating window. For example, if the first wavelength in an operating window, having n wavelengths, is set at a desired value precisely, an inaccuracy of dxcex in channel spacing will accumulate across the entire operating window, resulting in a significant offset of (nxe2x88x921)xc3x97dxcex for the last wavelength in the operating window. In addition, this paper also fails to provide peak position control during the device fabrication.
U.S. Pat. No. 5,408,555 (the ""555 patent) entitled xe2x80x9cPolarization Insensitive Wavelength Multiplexing 2xc3x972 Fibre Couplers,xe2x80x9d by Fielding et al., added an additional step of requiring continuous monitoring of the twisting process to enable termination of the manufacturing process at a particular moment, i.e., providing a relatively high level of precision in the spectral positioning of the minimum and maximum power transfer wavelengths for one of the principal planes of polarization of the coupler. However, the method described in the ""555 patent is complicated and time consuming because a polarized light source and a polarization controller are required. In addition, this method requires numerous torching steps during the polarization adjustment process, thereby increasing the insertion loss of the device. Furthermore, due to the use of a single-wavelength light source, the actual channel spacing and the polarization states across the entire desired wavelength range of the MWDM are not measurable. Therefore, this method is only suitable for fabricating WDMs, not MWDMs.
A problem associated with the long-tapered couplers is that when the fiber is pulled longer and longer and the passband spacing becomes narrower, moisture on the surface of the fibers causes the passband wavelength to drift. The light wave is generally confined to the core region of an optic fiber which has a round core region, before it is stretched. However, when two fibers are fused and stretched, the core region of the optic fibers at the fused region becomes smaller. The light passing through the optic fibers now spreads to the entire cladding region which is in direct contact with air, rather than being confined to the core region of the optic fibers. Moisture, including organic or inorganic vapor, on the surfaces of the optic fibers, causes the boundary condition to change, which in turn causes the wavelength peak to drift, making the system unreliable. The wavelength drift caused by moisture is especially critical in a DWDM because DWDM has very narrow channel spacings. Therefore, a small amount of drift in the wavelength can cause the system to be unstable and unreliable.
An additional problem associated with a fused coupler WDM with narrow channel spacing is sensitivity to temperature fluctuations. As temperature increases in the fused-fiber WDM, the refractive index of the fused-fiber portion increases due to the refractive index dependence on the temperature of the fused silica, which is approximately 6xc3x9710xe2x88x926/xc2x0 C. This causes a longer optical path inside the coupling region of the WDM. These temperature-induced shifts normally do not adversely affect conventional wide-band WDMs, which typically have channel spacings of 50 nm or more. However, with DWDMs, typically having channel spacings of 1 nm or less, such wavelength shifts can pose significant problems with transmission performance.
One way to produce a fused coupler is to use a fusion machine to heat and stretch the fibers. A conventional fusion machine comprises two identical chucks, both sitting on slides. The chucks hold the optical fibers to be coupled while a torch heats the fibers held between the chucks. The fibers are then stretched and fused together. The output optical power or spectrum is monitored during the stretching and fusing process. The stretching and fusing process is terminated when a predetermined output is obtained. The conventional fusion machine is incapable of twisting the fused fibers.
Accordingly, a structure and method are desired which provide a reliable polarization-independent environmentally stable optical fiber narrow band multi-window wavelength division multiplexer based on BTF coupler technology.
The present invention provides a multi-window dense wavelength division multiplexer (MWDM) which improves stability and performance in MWDMs by utilizing biconical tapered fusion (BTF) coupler technology, elastic twisting, a hermetic seal, passive temperature compensation for the couplers, coupler mounting scheme, and ultraviolet (UV) radiation exposure of the coupling region of the coupler and a method of making such a MWDM.
In accordance with the present invention, a pair of optic fibers is twisted along the coupling region after the fusion process to obtain a polarization-independent MWDM. The number of turns depends on the length of the fused coupling region and the wavelengths of the incoming light. By utilizing elastic twisting, a wider modulation envelope and more passbands of approximately equal amplitude with precise passband spacing are obtained. In one embodiment, the coupler is hermetically sealed to eliminate the wavelength drifting caused by moisture, allowing a stable MWDM to be produced.
A fusion machine is provided to perform the elastic twisting. The fusion machine comprises a fiber twisting device which includes a latch that locks a first chuck in a stationary position. The first chuck, together with a second chuck holds the fibers in place. The first chuck and the second chuck are positioned on the same plane so that the fibers are held in a horizontal position. The fiber twisting device and the second chuck are on sliders so that the distance between the first and the second chucks may be adjusted. The latch is released and turned so that the first chuck can be turned by turning a thumbwheel, until a predetermined polarization curve is obtained, i.e., by observing the output of a spectrum analyzer.
In one embodiment, the coupler is mounted on a substrate with a larger thermal expansion coefficient than the thermal expansion coefficient of the coupler to adjust the tension of the coupler, thus counteracting the intrinsic thermal drift of the coupler. In another embodiment, the coupler is mounted on the substrate using soft adhesives in conjunction with rigid adhesives, to improve mechanical shock protection.
In one embodiment, the coupling region of the coupler is exposed with UV radiation to change the refraction index of the coupling region, hence changing the passband wavelength of the coupler.
This invention will be more fully understood in light of the following detailed description taken together with the accompanying drawings.